1. Technical Field
The present invention relates to an arrangement for wheel suspension in a vehicle including a vehicle frame that bears at least two bogie axles that in turn each bear a wheel axle. There is also a first bogie element and a second bogie element that extend essentially in the longitudinal direction of the vehicle and are suspended in the vehicle frame.
2. Background Art
In connection with transporting heavy loads, for example in construction work, use is often made of vehicles of the frame-steered dumper type. Such vehicles can be driven with large and heavy loads in areas where there are no roads, which may be the case in, for example, transport operations connected with road and tunnel construction, sand pits, mines and similar environments.
Conventional frame-steered dumpers of this type usually include of a front vehicle part known as an engine unit, and which includes a front frame that bears an engine and a front wheel axle. The dumper also has a rear vehicle part in the form of a load unit with a rear frame that bears a front bogie axle and a rear bogie axle upon which the respective wheel axles are borne. The vehicle is normally adapted so as to be changed over for operation with different wheel combinations, in which connection it is possible for all six wheels on the three axles to be driven depending on the current operating conditions. A frame-steered dumper can weigh on the order of 15-30 tons and can be loaded with weight on the order of 20-35 tons.
In a frame-steered dumper, the frame of the engine unit is coupled together with the frame of the load unit via a special steering coupling that allows the engine unit and the load unit to be rotated in relation to one another about an imaginary longitudinal axis; that is to say, an axis extending in the longitudinal direction of the vehicle. This steering coupling also provides a vertical bearing for steering the vehicle. In this way, the engine unit and the load unit are allowed to move essentially independently of one another. This reduces the stresses that act on the vehicle, particularly when driving on difficult terrain.
A frame-steered dumper is normally equipped with a diesel engine and an automatic gearbox with, for example, six forward gears and two reverse gears. For braking the vehicle, use is made of a service brake that is suitably of the hydraulic type, and which is divided into two brake circuits, a first brake circuit being intended for the engine unit and a second brake circuit being intended for the load unit. As mentioned above, the dumper also includes three wheel axles and a bogie arrangement for suspension of the two bogie axles of the load unit. In this connection, the purpose of the bogie arrangement is to provide a suitable springing function for the dumper, with good comfort for the driver of the vehicle and damping of excessive swaying movements. The latter requirement is especially important for safety reasons when driving with a loaded vehicle.
Use is normally made of a bogie construction in the load unit that has two bogie elements in the form of bogie beams which are arranged extending in the longitudinal direction of the vehicle. Each bogie beam is arranged along its respective side of the load unit, extending essentially parallel to two corresponding frame beams that, together with transverse beams, form the abovementioned rear frame. Moreover, each bogie beam is pivotably suspended in the respective frame beam via swivel joints which allow a pivoting movement about an imaginary axis extending essentially transversely to the longitudinal direction of the load unit.
The front end portion of each bogie beam rests against the respective end portion of the front bogie axle, while the rear end portion of each bogie beam rests against the respective end portion of the rear bogie axle. The bogie beams act against the bogie axles via intermediate relatively rigid rubber elements with spring action. These spring elements are typically firmly connected to both the bogie axle and the bogie beam concerned.
The purpose of the bogie arrangement described above is to provide a suitable distribution of the load concerned over each bogie axle. To this end, the bogie arrangement is constructed so as to allow a diagonal bogie movement, that is to say a state in which the bogie axles are positioned with a different degree of inclination (compared with the horizontal plane) in relation to one another. In this way, the load that is carried by the vehicle can also be distributed between the four wheels on the load unit. The bogie arrangement is also designed in order to provide suitable springing in the vertical direction and a suitable degree of rigidity in the swaying direction of the vehicle.
During use of a frame-steered dumper, for example during construction work, there is a general requirement to be capable of driving as quickly as possible with the dumper. This applies especially when the dumper is driven without load. In order for it to be possible to maintain a high speed with a load unit that is not loaded, it is then particularly when driving on uneven ground desirable to have soft springing in the vertical direction. This affords good comfort for the driver of the vehicle. Otherwise, at high speeds with an unloaded load unit and rigid springing in the vertical direction, a problem may arise on account of the load unit having a tendency to bounce and rock, which of course has a negative effect on driver comfort. By making the springing weak in the vertical direction, this problem can therefore be eliminated.
In a frame-steered dumper as described above and having weak springing, however, another problem is encountered when the vehicle is then driven with load. To be more precise; in the form of inadequate roll rigidity of the vehicle or too low of a degree of rigidity against swaying movements. This can be observed, for example, when driving on bends. Weak springing in the vertical direction in an unloaded vehicle therefore acts counter to the requirement with regard to high roll rigidity of a loaded vehicle.
On the other hand, it can be said that sufficient rigidity against rolling movements is obtained by making use of relatively rigid springing. But rigid springing then leads to poor comfort when the vehicle is unloaded. Moreover, more rigid springing results in an increase in the dynamic forces that act against the ground, which in turn increases the wear on the road.
The abovementioned problems also apply in connection with trucks and towed vehicles, where inadequate rigidity in the rolling or swaying direction can lead to the risk of accidents and excessive rigidity can result in road damage and wear.
The problem of combining weak springing with sufficient roll rigidity can be solved in wheel suspensions based on single axles by means of a stabilizer that normally consists of an essentially U-shaped strut that is arranged so that it connects both wheels on the same axle to the frame or chassis of the vehicle. The end portions of the strut are then suitably connected to the springing system of the vehicle, while the central portion of the strut is connected to the vehicle chassis. When, for example during cornering, the vehicle sways, the springing on the outer side of the vehicle is compressed whereas the springing on the inner side of the vehicle is extended. This means that the stabilizer is twisted about its longitudinal axis, the torsional rigidity of the stabilizer then defining the sway rigidity of the vehicle. By suitably matching the dimensions and the torsional rigidity of the stabilizer, the arrangement contributes to better roadholding of the vehicle.
In connection with a bogie arrangement in a frame-steered dumper of the type mentioned above, however, a conventional stabilizer will counteract the abovementioned diagonal bogie movement, which is a disadvantage. On road vehicles, this could be accepted, but it is a serious limitation for cross-country vehicles, because the resistance to the diagonal bogie movement can lead to reduced pressure of some wheels on the ground, which then tend to slip or skid.
One solution to the abovementioned problem is to use a hydraulic arrangement for wheel suspension. This can then be adapted to control the springing function in the vertical direction and the roll rigidity independently of one another by means of hydraulic devices. These devices can in turn be operated by means of a control unit depending on the current operating situation. A disadvantage of such a solution, however, is that it is very expensive.